Based on the comments below, what would you say is the minimalist f number needed to shoot saying somewhere like the grand canyon, machu picchu or mountains?

"When shooting a normal landscape image it is normal to attempt to keep as much of the image in focus as possible.

This means selecting a small Aperture (remember the larger the number the smaller the actual Aperture) to ensure that you end up with a large depth of field. This will ensure that parts of the image that are both close and far away from you have a good chance of being somewhat in focus.

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On the assumption that your lenses can produce essentially diffraction limited resolution at the f-numbers we are considering (I haven't time to check that assumption) all you need to do is calculate your pixel area which for the XSi is, if I've got my sums right, about 27 µm². That corresponds to about the pixel size of the Nikon D2X on the interactive chart on this page. Once you get the hang of making the chart work you can see the size of the Airy disk for various f-numbers. Ideally, with D2X highlighted, somewhere between f/5.6 and f/8 is optimum while maintaining good depth of field but you could probably stretch to f/11 without noticing any softening, especially with a bit of judicious sharpening in post-processing.

* DLA (Diffraction Limited Aperture) is the result of a mathematical formula that approximates the aperture where diffraction begins to visibly affect image sharpness at the pixel level. Diffraction at the DLA is only barely visible when viewed at full-size (100%, 1 pixel = 1 pixel) on a display or output to a very large print. As sensor pixel density increases, the narrowest aperture we can use to get perfectly pixel sharp images gets wider.

DLA does not mean that narrower apertures should not be used - it is simply the point where image sharpness begins to be compromised for increased DOF and longer exposures. And, higher resolution sensors generally continue to deliver more detail well beyond the DLA than lower resolution sensors - until the "Diffraction Cutoff Frequency" is reached (a much narrower aperture). The progression from sharp the soft is not an abrupt one - and the change from immediately prior models to new models is usually not dramatic. Check out this specific diffraction comparison example using the ISO 12233 chart comparison tool. The mouseover feature will show you the degradation at f/11 compared to f/5.6.

That isn't quite the right question being asked or answered. You're not generally looking to get the sharpest image. What you want is for as much of the image as possible to be "sharp enough".

While diffraction limiting is contributing factor, that ONLY applies to the focal plane. Everywhere else, by definition, is less sharp as it is out of focus.

In practice, depth of field calculations are more relaxed than pixel perfect levels. Personally I'd suggest not worrying about diffraction softness unless it gets too extreme. Just use a suitable aperture for the level of depth of field you want to realise. It depends on the focal length of the lens too, so you would need smaller apertures as the focal length gets longer.